World coordinates of objects
The location of an object is determined by the location of it's Origin.
Most likely all your object origins are at 0,0,0, and not in the centers of your objects.
To move the origins to your objects, select everything (A) and press CtrlShiftAltC> Origin to geometry.

I don't think there is any native way to display units on the grid.

I'm not quite sure what you mean by "systematically", but you could try snapping.